Composites are often made to create a beneficial mix of the properties of dissimilar materials that are unobtainable in a single homogenous material. In the field of transducers for example, it can be advantageous to use a piezoelectric ceramic-polymer composite, rather than a monolithic block of of piezoelectric material such as lead zirconate titanate (PZT). Piezoelectric composites consist predominantly of a polarizable phase embedded in a non-polarizable material.
These composites have many advantages over traditional monolithic piezoelectric ceramics including: (i) lower densities resulting in acoustic impedance closer to those of the human body, water, etc., thereby eliminating the need for an acoustic matching layer; (ii) low dielectric constants resulting in a high piezoelectric voltage constant g; and (iii) ease of conformability to the shape of the backing material of the composite. Such composite piezoelectric transducers and methods for their production, are described, for example, in Composite Piezoelectric Transducer; R. E. Newnham et al.; Materials in Engineering, Vol. 2, December 1980, Pages 93-106, which is incorporated herein by reference.
Three composite designs that have been particularly successful are composites with 2-2, 3-3 and 1-3 connectivity, in which piezoelectric ceramic sheets or network or rods are aligned in the poling direction of the composite and embedded in a matrix of a suitable polymer. In the case of the 2-2 composite, both the ceramic and polymer phases are two-dimensionally self-connected throughout the composite. The stiff ceramic phase supports most of the stress applied in the direction of its alignment, yielding a high piezoelectric charge coefficient d, while the composite has a low density and dielectric constant.
In the 3-3 composite, both the ceramic and the polymer phases are continuously self-connected in all three dimensions. The geometry and structure yields a high piezoelectric charge coefficient d and exhibits superior properties over single phase piezoelectrics.
In the 1-3 composite, the ceramic phase is one-dimensionally self-connected through the composite, while the polymer phase is three-dimensionally self-connected. For some applications, the 1-3 composite yields superior properties to those described above for the 2-2 composite due to the lower density and dielectric constant.
A common and convenient method for making 2-2 and 1-3 composites is to start by cutting parallel slots into a monolithic piezoelectric ceramic block. The slots are then filled with a polymer. The aforementioned method is known as the "dice and fill" method and is described in PZT-Epoxy Piezoelectric Transducers: A Simplified Fabrication Procedures, H. P. Savakus et al.; Materials Research Bulletin, Vol. 16, 1981, pages 677-680, which is incorporated herein by reference.
Two common methods for making piezoelectric composites with 3-3 connectivities are the replamine and burned-out plastic spheres, or BURPS, processes. The replamine process is a lost wax method with coral as a starting material. The coral is machined to the desired geometry and then back-filled with wax. After the wax is hardened, the coral skeleton is leached away using hydrochloric acid leaving a wax negative of the original coral template.
The wax negative is back-filled with PZT slurry and dried. The wax is subsequently burned off at a moderate temperature, leaving a coral-type structure of PZT. The structure is then sintered and back-filled with a desired polymer, usually a non-polarizable one, to make the final structure. The structure is then poled using conventional poling or corona discharge technique. The replamine process is described in Flexible Composite Transducers, D. P. Skinner et al.; Materials Research Bulletin, Vol. 13, 1978, pages 599-607 and is herein incorporated by reference.
In the case of the BURPS process, plastic spheres and PZT powders are mixed in an organic binder. After binder burn-out and sintering, a porous PZT skeleton is formed and later back-filled with polymer to form a 3-3 composite. The BURPS process is described in Simplified Fabrication of PZT/Polymer Composites, T. R. Shrout et al.; Materials Research Bulletin, Vol. 14, 1979, pages 1553-1559 which is also incorporated herein by reference.
Although, as mentioned above, piezoelectric composites typically consist of a polarizable phase embedded in a non-polarizable phase, there is a need to develop efficient methods for the manufacture of composites having multiple polarizable and/or non-polarizable phases. Also, there is a need to develop more efficient methods for the manufacture of piezoelectric composites having decreased size and periodicity of the polarizable phase or phases. Composites having these properties have been identified as a key area of transducer development.
Moreover, there has been a drive to create so-called "smart" materials. Smart materials are described in Smart Ceramics; Newnham et al., Ferroelectrics, Vol. 102, pp. 259-266 which is incorporated herein by reference. A smart material senses a change in the environment, and, using a feedback system, makes a useful response. It includes both a sensor phase and an actuator phase. A very smart material can tune its sensor and actuator functions in time and space to optimize its behavior. Tuning of a very smart material can be accomplished by using a multitude of polarizable phases.
It is well recognized in the art that the dice and fill, replamine and BURPS methods of forming composites have several limitations in meeting the above stated needs. For example, in the case of replamine process, the design, volume fraction and the scale of the structure depends on the starting coral template and can not be altered. The BURPS process too, lacks the flexibility of the design and fabrication of a fine scale 3-3 PZT composite structure with a controlled volume fraction.
In all of these processes, varying the ceramic volume content across the composite is not practical. Additionally, these techniques are designed for monolithic ceramics, ruling out the possibility of efficient and effective manufacture of multiphase ceramic composites.
Recently, solid freeform fabrication techniques have been developed for producing three-dimensional articles without the need for molds, dies, or other tooling. One such technique, commercialized by Stratysys.TM., Inc. of Eden Prarie, Minn., builds solid objects layer by layer from polymer/wax compositions by using computer-aided design (CAD) software programs. According to the technique, which is described in U.S. Pat. No. 5,121,329 and is incorporated herein by reference, a flexible filament of the polymer/wax composition is fed by a pair or counter rotating rollers into a dispensing head which includes a liquifier and nozzle outlet. Inside the liquifier, the filament softens and melts at a temperature just above its melting point.
As the counter-rotating rollers continue to advance the solid filament into the liquifier, the force of the incoming solid filament extrudes the molten material out from the nozzle where it is deposited on a build platform positioned in close proximity to the dispensing head. The CAD software controls the movement of the dispensing head in the horizontal X-Y plane and controls the movement of the build platform in the Z direction. By controlling the processing variables, the extruded bead, called a "road", can be deposited layer by layer in areas defined from the CAD model, leading to the creation of an object that is a three-dimensional depiction of the CAD model.
Although the fused deposition technique is explained in detail above, other techniques, including, but not limited to, stereolithography, selective laser sintering, sanders prototype, and laminated object manufacturing can be used in this invention. In stereolithography, for example, as described in U.S. Pat. No. 4,929,402, which is herein incorporated by reference, an ultraviolet ray curable polymer is used as a feed material and a computer controlled and focused beam of ultra violet rays is used to fabricate three dimensional objects.
In selective laser sintering, which is described in U.S. Pat. No. 4,938,816 and which is hereby incorporated by reference, a laser curable polymer is used as a feed material and a computer controlled and focused laser beam is used to fabricate three dimensional objects. In Sanders.TM. Prototype technology, an ink jet printing process is used where a thermoplastic polymer is used instead of an ink. Three-dimensional objects are built by depositing layer upon layer of thermoplastic polymer on a computer controlled fixtureless platform.
In Laminated Object Manufacturing (LOM), sheets of paper, polymer, or ceramic materials are deposited on top of each previous layer and a computer controlled laser beam is used to cut the sheet of material to make the three dimensional object.
This invention takes advantage of the aforementioned solid freeform fabrication technology to overcome many of the limitations of electronic ceramic composite manufacturing technology. Moreover, the disclosed invention makes possible the efficient manufacture of such composites with phase geometries that have previously been impossible and/or impractical.